The purpose of this research is to develop a model system to study gene expression in normal and malignant animal cells. The system is the mouse plasmacytoma and its normal cognate, the B-cell. The success of this study is highly dependent on the preparation and precise characterization of two specific molecular probes--the immunoglobulin mRNA and its complementary DNA (cDNA). Both probes can be highly radiolabeled with iodine-125 in vitro (about 10 to minus the 9th power cpm/ug. The mRNA will be prepared from different mouse myeloma tumors that have well-characterized, specific immunoglobulin products, thus facilitating correlations of known amino acid sequences with unknown nucleotide sequences of mRNA and cDNA. After establishing the biological activity and purity of the mRNA, highly discriminating cross- hybridization experiments will be performed between the mRNA and cDNA presumably representative of different types of immunoglobulin variable and constant genes, in order to establish the correlation above. Oligo (dT)-initiated cDNA product appears to be representative of almost exclusively the constant gene of the light chain, while oligo (dG)- initiated cDNA product might be representative of the variable gene. These mRNA and cDNA probes will then be used in hybridization studies to: (1) determine the total number of constant and variable genes; (2) show their chromosomal and intracellular location by in situ hybridization; and (3) study specific areas of RNA transcription, such as specific chromatin template activity and specific heterogenous nuclear RNA transcript composition.